Application of response surface methodology to optimize high active Cu-Zn-Al mixed metal oxide fabricated via microwave-assisted solution combustion method

Abstract

The present work deals with evaluation of decisive parameters on performance and structure of Cu0.4Zn0.6Al2O4 catalyst. In this respect, the experiments were designed by the response surface methodology (RSM), considering the main parameters: the amounts of urea and ammonium acetate and the activity were assessed in esterification reaction. The results of statistical analysis showed that the proposed second-order equation is highly compatible with experimental data and the interaction between the ratios of both fuel types is highly effective as well independent variables. The catalyst synthesized with the fuel content higher than the stoichiometric ratio (67% urea and 45% ammonium acetate) has the highest catalytic activity in the esterification reaction, where the efficiency of about 98.2% is obtained in the esterification reaction under the conditions of 180 °C, methanol to oleic acid molar ratio of 9, 3 wt% of catalyst and 6 h of reaction time. The sample fabricated with 67% urea and 45% ammonium acetate as fuel (more than stoichiometric ratio) presented the highest activity that could convert 98.2% of oleic acid to ester. Characterization of the catalysts showed that the particle size reduced from about 18 nm for the catalyst synthesized with single fuel to 12 nm for that synthesized at fuel rich condition. Moreover, the unit cell size was meaningfully changed by using combined fuel that can be related to incorporation of simultaneous Zn and Cu for preparation of aluminate structure. As well as use of combination urea and ammonium acetate led to increase the textural properties (surface area from 4.76 m2·g−1 to 8.39 m2·g−1 and mean pore size from 2.1 nm to 8.9 nm). The results can confirm formation of dopant structure of Zn/CuAl2O4 in which a few amount of single phase was detected.